Can't join dedicated server

My friend rents a dedicated Server for Space Engineers. He and another friend are able to join without a problem, they can also see the server in the server browser. I can't. I can neither see the server in the game server browser nor in the steam browser. When I try to connect directly, nothing happens. When I try join via Steam's "join this friend's game" option, it says, the server doesn't respond. It then also shows the wrong IP, so it makes sense I can't use that feature.